Continuous track vehicles (also known as “track laying” vehicles) are well known and widely used. The present invention relates to such vehicles and has particular applicability to military tanks. The construction of military tanks and their tracks is well known and is described for example in Cory, U.S. Pat. No. 4,765,694, Kent, U.S. Pat. No. 6,080,493, Soucy et al, U.S. Pat. No. 7,416,266, Gonzalez, U.S. Pat. No. 7,445,299, and Touchet et al, U.S. Pat. No. 5,264,290, the disclosures of all of which are hereby incorporated by reference.
Briefly, a military tank typically includes two tracks, one on each side of the vehicle. Each track has a road-engaging side and an inner side which engages a set of road wheels, as well as a drive sprocket at the forward or rearward end of the vehicle, an idler wheel at the other end of the vehicle, and support rollers along an upper reach of the track above the road wheels. The track itself is formed of track shoe assemblies. Each assembly typically includes two track shoes, spaced laterally from each other and separated by a center guide which guides the track around the road wheels, drive sprocket, idler wheel and support rollers. Each track shoe carries an elastomeric inwardly-facing road wheel pad bonded to its inner face and an elastomeric outwardly-facing road pad removably mounted to its outer face. The pads are generally rubber, although composite pads have also been proposed for the road pad.
Each track shoe assembly also includes two pin assemblies extending laterally across and beyond the track shoe assembly. The pin assemblies hold the two track shoes of each shoe assembly together in alignment with each other, and the ends of pin assemblies of adjacent shoe assemblies are connected by end connectors. In one modern arrangement, the center guide is mounted on the pin assemblies of adjacent shoe assemblies, so that a projection part of the center guide straddles adjacent shoe assemblies, rather than being centered between the track shoes of a single shoe assembly. Although metal bushings have been used in tractor-type continuous track vehicles, as described for example by Wright, U.S. Pat. No. 3,313,578, large continuous track vehicles, such as military tanks, have used rubber bushings.
In a military tank or the like, each pin assembly includes a hollow pin in the form of a steel shaft drilled out to reduce its weight. Multiple rubber bushings are bonded to the exterior of the pin. Bonding is typically produced by injection molding the rubber onto the pin, and then vulcanizing the rubber. The number of bushings shown in the patent literature is generally two: one for each track shoe. In practice, seven rubber “donuts” are bonded to the pin for each track shoe, giving fourteen rubber bushings for each pin assembly, in an arrangement similar to Krotz, U.S. Pat. No. 2,430,573. This arrangement allows the rubber to compress and spread laterally (axially of the pin) when the pin assembly is forced into the shoe assembly. The rubber conventionally has a nominal hardness of about 75 A on the Shore hardness scale before it is compressed by about forty percent when pushed into the sleeve of the track shoe assembly. The force required to push the pin into the shoe assembly requires a thirty- to fifty-ton press. For many years, the art has taught that, “Such compression must be sufficient so that there is no rotation between the bushing and the outer sleeve portions or the pin upon bending of the track during use.” (Cory, U.S. Pat. No. 4,840,438 at col. 2, lines 9-12). This arrangement insures that “relative rotation therebetween [between the pin and the track shoe] along the length of the pin assembly is controlled by elastic deformation of the resilient bushing.” (Id. at col. 13, lines 41-43). Because the rubber bushings are bonded to the pin, there has been no need to machine the pin to close tolerances, and the pin may have a variation in diameter of as much as 0.020″ (0.5 mm) and may deviate substantially from being linear. Conventional pin assemblies incorporating compressed bushings are disclosed in Knox et al, U.S. Pat. No. 1,955,751, Lamb, U.S. Pat. No. 1,973,214, Knox et al, U.S. Pat. No. 2,089,210, Knox, U.S. Pat. No. 2,301,954, Saurer et al U.S. Pat. No. 2,332,976, Krotz, U.S. Pat. No. 2,430,573, Reynolds et al, U.S. Pat. No. 3,357,750, Huhne et al, U.S. Pat. No. 4,139,241, Fix, U.S. Pat. No. 4,165,906, Ruddell, U.S. Pat. No. 4,195,887, and Wiesner, U.S. Pat. No. 5,749,634, the disclosures of all of which are hereby incorporated by reference.
Cory, U.S. Pat. No. 4,840,438, discloses that the extreme compression required by the conventional compressed bushings creates assembly problems and also “results in a greater resistance to the required bending during use and a consequent greater power requirement for driving of the associated vehicle.” To solve this problem, Cory molds and bonds the rubber bushing in situ to the pin and to a pair of sleeve sections having a positioning flange which fits into a groove in the associated track shoe when the bushing is compressed about 5% and pressed into the shoe assembly. The flange and groove prevent rotation of the pin assembly with respect to the shoe assembly, thereby insuring any relative rotation of the pin and shoe is controlled by elastic deformation of the rubber. Cory's arrangement, however, adds considerable complexity and additional manufacturing requirements, thereby substantially increasing the cost of the track.
Particularly with heavy continuous track vehicles, such as military tanks, the lifetime of the track is very short, frequently a few hundred miles of use. When the track fails, the vehicle may become unusable. The failure of tank tracks creates a huge financial cost and may cause logistical issues and personnel safety issues. The problem is particularly acute with tanks of the size of a Bradley Fighting Vehicle having a weight of greater than about twenty-five tons, an M-60 battle tank having a weight greater than about fifty tons, or an Abrams M-1 battle tank having a weight greater than about fifty tons to about seventy-five tons or more. The problem has long been known, as witnessed for example by Ruddell, U.S. Pat. No. 4,195,887, filed 1978 (“The compressive and shear loads placed on the elastomeric bushings far exceed the capabilities of present elastomers to withstand these loads and the elastomeric bushings wear out and must be replaced.”), Cory, U.S. Pat. No. 4,840,438, effectively filed 1985 (“The constantly increasing weight and speed of track laying vehicles has resulted in a decrease in the track life and a constant increasing need for track maintenance by either repair or replacement.”), or Kent, U.S. Pat. No. 6,080,493, filed 1998 (“A complete set of tracks for a U.S. Army M-1 Abrams tank can cost as much as $100,000.00 and may only last from 300 to 2000 miles.”).
The problems with track life are two-fold. Although the inner road wheel pads apparently exhibit little wear, the outer road pads are subject to substantial wear and to hazards such as puncturing. They are therefore made to be field replaceable. Second, and more seriously, the rubber bushings rapidly fail. In practice, the bushings generally fail before the road pads, so the entire track is frequently replaced before the road pads can be field replaced. Replacing the entire track requires removing the tank to a repair facility via a tank carrier vehicle.